This invention relates to a modulator and demodulator apparatus particularly adapted to operate with a fiber optic information transmission system.
Fiber optic systems using glass fiber guides are extensively employed. In regard to such systems, as for example, a communications system, the objective is to transfer information from one point to another. The transfer of information is accomplished by modulating the information onto a carrier wave which is then transmitted or propagated to the receiving site where the information is recovered by demodulation. The principal advantage in communicating with optical frequencies is that one obtains a potential increase in information and power that can be transmitted. Thus the field of optical communications has been rapidly developing.
A typical optical communication system has one or more data sources where the output is modulated onto an optical carrier. This carrier is then transmitted as an optical light field or a beam through the optical channel which is for example an optical fiber. At the receiver, the field is optically collected and processed by the use of a photodetector or other device.
In regard to such systems, sources for light transmission include semiconductor lasers and LED devices. LED's are adequate for data links and are extensively used in the field of optical communications. As such, longer wavelength LED's are also employed for major transmission roles. In any event, there is a problem with LED devices in that they exhibit nonlinearity in operation particularly when they are operated at high current levels to produce high power output. Thus in prior art systems employing LED devices in conjunction with the modulator, one had to severely limit the extent of modulation and the power range over which the LED operated in order to assure linear operation. These difficulties are avoided with the modulation and demodulation apparatus of the present invention.
It is an object of the present invention to provide an improved modulator/demodulator apparatus useful in fiber optic systems employing LED devices for transmitting a modulated signal over an optical communications channel such as an optical fiber.